The present invention generally relates to a duplex recording sheet transport control apparatus, and more particularly to a duplex recording sheet transport control apparatus used in a printer or copying machine which makes double-sided prints.
As an example of the conventional apparatus, a single-side printing of a sheet carried out with a laser printer 1 shown in FIG. 21 will be described as follows. Recording sheets 6 are supplied from an upper paper cassette 2 or lower paper cassette 3 into a path indicated by the arrow A through a paper feeding device 4 or 5 (in this case, an upper paper feeding device 4 is used). These paper cassettes are attached removably to the printer and contain supply papers. The sheets 6 are transported to a latent image carrier including a photosensitive body 8 on a drum in a timing which is controlled with a pair of registration rollers 7. The photosensitive body 8 is rotated with the drum in a counterclockwise direction and at the same time the surface of the body is charged by an electrostatic charger 9. And laser beam L from a laser beam optical system 10 is applied to form an electrostatic latent image on the photosensitive body.
This latent image is changed with a toner into a visible image when the sheet passes through a developing device 11. The visible image is transferred through a transfer/separation charger 12 to the recording sheet 6 which is transported to the photosensitive body 8, and the sheet 6 in contact with the photosensitive body 8 is electrostatically separated. Then, the recording sheet 6 is transported to a fixing device 13 so that the visible image on the sheet 6 is fixed, and the sheet 6 is carried into a path indicated by the arrow B in this figure to a paper outlet 14.
Meanwhile, from the photosensitive body 8 after the visible image is transferred to the recording sheet, residual toner is removed by a cleaning device 15 having cleaning blades, and the removed toner is collected in the cleaning device 15.
Next, an overall description of the duplex recording or double-sided printing carried out for recording sheets 6 will be given below. As shown in FIG. 22, the single-sided prints (printed on the reverse side only) are further transported into a duplex apparatus 16 which is installed on the upper portion of the laser printer 1. In this apparatus the direction of transporting the sheet is reversed, and the sheet is again sent to the printer 1 which carries out again the single-side printing of the sheet on the opposite side (printed on the front side of the sheet in this case) and transports it to the paper outlet 14.
As shown in FIG. 22, the duplex recording apparatus 16 includes a transport system 16A and a switchback system 16B. The transport system 16A includes a first drive motor 20 for driving the rotation movements of a first transport roller 17, a second transport roller 18 and a third transport roller 19, a first paper sensor 21 provided on the upstream side of said first transport roller 17 along the sheet transport path for detecting the sheet front edge, a selector 22 used for changing the direction of transporting the sheet from a paper outlet roller 32 within the printer 1 to select a first paper transport path 101 with the rollers 17 through 19, and a solenoid 23 for driving said selector 22.
The switchback system 16B of the duplex recording apparatus 16 includes a switchback roller 24, a second drive motor 25 for driving rotary movement of said switchback roller, a switchback paper tray 26 for temporarily storing the sheet sent from the above-described first paper transport path 101, a selector 27 used for changing the direction of transporting the sheet from said switchback paper tray to select a second paper transport path 102 for the inverted sheet, and a solenoid 28 for driving said selector.
In FIG. 22 the position 30 indicates a first paper waiting position and the position 31 shows a second paper waiting position. A single-sided print (recorded on reverse side only) transported by the outlet roller 32 of the printer 1 to a third transport path 103 is further routed to the first transport path 101 by means of the selector 22. When the first paper sensor 21 senses the front edge of the sheet, it changes from the OFF state to the ON state to serve as a trigger to start the operation of the first and second drive motors 20 and 25. The amount of revolutions for these motors having stepping motors, or the sheet transport distances, are controlled with a magnetizing step number by a CPU (not shown) of the duplex recording apparatus control unit.
FIG. 23 is a diagram showing the magnetizing step numbers xi (i=1 to 4) for the first drive motor 20 which starts operation when the first paper sensor 21 changes from the OFF state to the ON state, as well as the magnetizing step numbers yi (i=1 to 4) for the second drive motor 25 which starts operation in the same manner. These magnetizing step numbers differ depending on the length of recording sheets, and are a fixed value determined primarily by the sheet type.
As is apparent from FIG. 23, in the conventional apparatus, the start timing (x1) is when the first paper sensor 21 changes from the OFF state to the ON state, and the first drive motor 20 temporarily stops operation (x2) when the sheet front edge reaches the first sheet waiting position 30.
Then the first drive motor 20 re-starts forward operation, and when the sheet front edge reaches the switchback drive motor 24 the second drive motor 25 starts operation (x3). When the sheet rear edge goes through the third transport roller 19, the first drive motor 20 stops operation (x4, y1). And the second drive motor 25 starts reverse operation with the sheet rear edge changing to the front edge (y2). While the second drive motor 25 stops, the sheet front edge reaches the second waiting position 31 (y3). Next, the second drive motor 25 re-starts operation to supply the next sheet to the printer 1, and then the second drive motor 25 stops operation.
In the conventional duplex recording apparatus, the timing of transporting the sheet from the first waiting position 30 to the switchback system 16B is determined based on the occurrence of the ON state in the sensor 21, and the re-starting of the first drive motor operation is performed simply after the OFF state of second drive motor 25 is sensed. Because of this, it is very difficult to adjust paper feed time intervals for a speedy printing. A paper feed time interval is referred to as a time period between paper feedings.